Breast cancer has long been recognized as the most common cancer diagnosed in women;[1] however, as of 2020, having accounted for 12% of all new cancer cases each year, the World Health Organization now claims it to be the most common cancer diagnosed globally.[2] By the end of 2022, it is estimated that in the United States there will be 287,850 new cases of invasive mammary carcinomas, alongside 51,400 new cases that are non-invasive. On a positive note, death rates have been decreasing in breast cancer since 2007 with a rate of 1% dropping every year between 2013 and 2018, which is thought to be a result of scientific advancements in treatment approaches and the earlier detection of cases due to a more widespread protocol in screenings conducted.[1] Despite significant advances in cancer research and drug development initiatives, the five-year relative survival rate in metastatic breast cancer remains at 27%.[3]
Before we get into the details of how breast cancer is posing a challenge to modern healthcare in our society, we need to understand the fundamentals of the subject. The human breast is primarily made up of two types of tissues: Glandular and stromal. Glandular tissues consist of milk-producing glands (lobules) connected to and drained by the ducts (the milk passages) and are surrounded by fat and fibrous connective tissue collectively named stromal.[4] The mammary gland is also one of the few organs in the human body that completes its developmental stages postnatally through a tightly regulated process of differentiation through stages of pubertal growth, pregnancy, lactation, and involution and a dysregulation occurring during any of these stages may contribute to breast carcinogenesis in patients.[5]
The majority of tumors that develop inside the human breast are non-cancerous, such as fibrocystic changes, in which accumulated packets of fluid in the glands make up cyst, and a scar-like connective tissue formation in the stroma cause lumpiness, tenderness, and/or breast pain for the patient.[5] According to its site, breast cancer can be classified into non-invasive and invasive subtypes, where the invasiveness is denoted by the breaking of the cancer cells through the ducts, and the lobular walls of the breast to invade the surrounding stromal tissue. Amongst the types of breast cancer that are non-invasive, ductal carcinoma in situ (DCIS) is the most common type, making up almost 90% of cases. Lobular carcinoma in situ (LCIS) on the other hand, develops within the milk-producing glands of the breast and is considered to be a marker for increased risk of malignancy in the future.[6] Genetically, breast cancer is classified into four types: Human epidermal growth factor receptor 2 positive (HER2+), which is also known as ERBB2 in humans; Luminal A, where the tumor is also stained positive in immunohistological assays for estrogen receptor (ER), may or may not have also been shown to express the progesterone receptor (PR) but is shown not to express the ERBB2, while also not portraying high levels of mitotic activity (Ki67 levels below 14%); Luminal B, where on top of the findings seen for luminal A, ERBB2 may also have been tested positive and the Ki67 value of the tumor (denoting mitotic activity) is seen above 14%[7] and finally the triple-negative type of breast cancer, where none of the receptors mentioned above (ER, PR, ERBB2) are seen expressed in the tumor and the mitotic activity is also seen high (Ki67 levels above 14%).[8] Understanding this taxonomy is key for reviewing the plethora of research that has been conducted on the topic and is essential to manage treatment approaches in clinical settings.
Breast cancer is known to be a multifaceted disease that involves a patient’s genetics and many environmental factors; however, the role of certain human hormones such as estrogen, progesterone, oxytocin, and prolactin in the propagation and the pathogenesis of the disease has long been a topic of research in breast cancer. Human prolactin hormone was for the first time seen to be potentially linked with breast cancer when in the 1990s, a study showed that the activation of the human prolactin receptor (PRLR) in transgenic mice induced mammary carcinomas.[9] Oxytocin receptor, on the other hand, a member of G-protein coupled receptors of the A/rhodopsin family is known to be found highly expressed in breast carcinomas ever since the late 1990s.[10-12] This mini-review article focuses on the roles of prolactin and oxytocin in breast cancer in relation to their receptor expression in the tumors, and the therapeutic indications of utilizing the scientific findings made in regard.
OXYTOCIN
Oxytocin is a cyclic nonapeptide with a C-terminal amide and a disulfide bond between its two cysteine residues it elicits many responses throughout the body, including its functions in both the central and peripheral nervous systems, regulating social bonding, and sexual activity, stress, maternal behavior, uterus contractions, and milk ejection.[13] The role of oxytocin in breast carcinogenesis has been an important topic for research in the field, especially since childbearing and breastfeeding have been found to be protective factors against breast cancer; processes during which it plays a crucial role.[14,15] It’s also found that people who have birthed no children at all during the course of their lives have a higher risk of developing breast cancer, compared to those who have given birth at one point.[16-18] Oxytocin is also produced through nipple stimulation, where it facilitates the alleviation of radical oxygen species from the acinar glands through distention, by the means of myoepithelial cell contractions.[16]
The physiologic aspect
During pregnancy, breast development picks up the pace and the mammary glands start to undergo complete remodeling in order to mature into a functional milk-secreting organ with the lactation cycle.[19] As weaning commences and the baby is introduced to solid foods after the age of six months, the glands then regress into a resting state called involution.[20] It is found through studies conducted on lactating rats, that oxytocin has a role in maintaining the status of the glands, delaying involution with influence on cell growth and differentiation.[21-24] In all breasts, whether they are neoplastic or in healthy condition, cells that are from both myoepithelial and epithelial origins produce and secrete oxytocin, creating local autocrine and paracrine loops to enhance the effects of mammotropic hormones, inducing proliferation seen both in vitro and in vivo settings,[25] and also, postpartum.[26]
Role in carcinogenesis
The research on this topic has not been conclusive and comprehensive enough to reveal the full depth of the involvement of oxytocin in breast cancer, but the findings thus far point to an elusive fact that oxytocin and its effects produced through its signaling pathways, in different mechanisms, both mitigate and promote the carcinogenic and metastatic processes of mammary tumors. Research conducted on MDA-MB-231 cell lines, which are of triple-negative breast cancer origin, demonstrated how overexpression of oxytocin receptors is linked to the migration and metastasis of cancer cells, while also facilitating epidermal growth factor (EGF) sensitivity.[27] In another study where oxytocin receptor overexpressing tumors were used in Gene Set Enrichment Analysis (GSEA), genes that enable the potential for the proliferation of cancer cells, disease progression, and ERBB2+ carcinogenesis were found upregulated. It was also deduced during the same study that oxytocin receptor overexpression created a set of circumstances on a cellular level that facilitates tumor growth and metastasis - this was found through the transplantation of oxytocin receptor overexpressing tumor fragments (E0771) into the fourth mammary glands of oxytocin receptor overexpressing and wild-type mice to check for tumor growth and tumor weight after 15 days-subsequently, results from the oxytocin receptor overexpressing mice were found to be much higher than what was observed in the wild-type samples.[28]
Another study conducted on mice, on the other hand, involved seven different groups of mice undergoing separate treatments of oxytocin administration, exercise training - which also enhances the endogenic oxytocin secretion in mammals - and atosiban administration (oxytocin antagonist) for two weeks. Results in this study and, the Western blot assays conducted to evaluate the Akt and ERK protein expression in the mice involved, showed that high plasma oxytocin levels served the purpose of decreasing the levels of Akt and ERK expression in samples, which are long-known to be proteins serving as parts of the Phosphatidylinositol-3-kinase (PI3K/AKT) and mitogen-activated protein kinase (MAPK) pathways; these are both involved in the regulation of growth, an increase in number, lifespan, and the potential to move and invade in their surroundings for cells, and they are both found activated in a wide range of human cancers. Inhibition of both of these proteins through exercise training or oxytocin administration is concluded to be correlated with the inhibition of tumor growth in the study at hand.[29]
There are in general more than ten cell lines of human breast cancer that express the oxytocin receptor on a protein or on a messenger ribonucleic acid (mRNA) level, including, but not limited to MCF7, MDA-MB-231, T47D, BT20, and BTEC, which is a non-cancer stromal cell line derived from the endothelial cells from human breast carcinoma sample.[30-32] A review of the effects of oxytocin analog administration on various human breast cancer cell lines in vitro[19] demonstrated that MCF7 cells incubated in fetal calf serum (FCS) without the presence of estrogen (E2) at 2.5% concentration levels, showed stimulation of cancer cell proliferation,[32] which was also seen together with the stimulation of cell migration in BTEC cells, incubated in a serum-free environment or an FCS solution on 10% concentration.[33] However, on cell lines such as T47D, MDA-MB-231, TS/A (mouse sample), and CMT-U27 (canine sample) a clear inhibition of cell proliferation was observed.[34,35] These contradictory results are potentially stemming from factors such as the variability of gene expression for oxytocin receptors that would be induced by hormonal levels, the composition of the serum samples used, and other environmental changes during different experiments conducted.[36,37]
PROLACTIN
Another important hormone that has proven itself to be a worthy highlight in breast cancer research in recent years is prolactin. Prolactin is a neuroendocrine hormone produced primarily in the adenohypophysis, by the lactotroph cells in the gland but also in the breasts, uterus, prostate, lymph cells, and placenta decidua.[38] It stimulates deoxyribonucleic acid (DNA) synthesis, epithelial cell proliferation, and more notably, breast development in human physiology and it exerts all its effects on the human body through its interactions with the prolactin receptor.[39] It is a key hormone in the mediation of breast tissue growth and differentiation as well as the process of lactation.[40] The prolactin receptor is a member of the cytokine receptor superfamily with three main domains: One located extracellularly for ligand binding, a short section that crosses the cell membrane and an extension into the cytosol meant for interaction with signaling molecules.[41] It is found upregulated in up to 80% of all breast cancer cells.[42] The prolactin receptor interacts with several signaling pathways like the JAK2/STAT5, MAPK, and the PI3K to promote the proliferation and survival of cancer cells as well as interact with their dynamics of cytoskeletal structures.[40] Through its receptor, prolactin reduces the efficacy of available treatments for breast cancer by antagonizing the cytotoxic effects of chemotherapeutic agents.[43] Signaling pathways like MAPK and PI3K constitute a major intermediate by which sex steroids interact with cytoskeletal structures, more prominently the actin backbone of the cell.[40] Similarly, the interactions of prolactin with these pathways allow the cancer cells to modify the spatial organization of these filaments including their links with membrane-anchoring structures, to achieve local and distant metastases, especially within T47D, ZR75-1 and MCF7 cancer cell lines.[44]
Large prospective epidemiologic studies, such as the Nurses’ Health Study[45] and the European Prospective Investigation into Cancer and Nutrition (EPIC) study,[46] showed how higher prolactin serum levels predicted an increase in breast cancer risk for the patient, independent of estrogen levels. Other epidemiological studies have shown a correlation between higher prolactin levels and higher mammographic density upon clinical inquiries, which is an important independent contributor to a significantly increased risk of getting diagnosed with breast cancer - especially for aggressive estrogen receptor-positive (ER+) type in postmenopausal women.[47-49] In addition to these effects of circulating prolactin, also locally elevated levels contribute to breast cancer development and progression by inducing ductal abnormalities and epithelial hyperplasia.[50]
Like many other cancer types, 80% of in vivo breast carcinomas examined in these handfuls of studies, induced by prolactin administrations, exhibit alterations of the Ras pathway, and copy number amplifications and activating mutations of KRAS; while the remaining 20% show elevated pAKT (phosphorylated RAC - α serine/threonine kinase, an indicator of poor prognosis in invasive breast cancer[51] levels, demonstrating consistency with mutations in the PI3K pathway. It is evident that, as seen through many studies conducted, prolactin induces proliferation and cell turnover in breast tissue.[52-55] However, it is also seen to influence epithelial stem cell or progenitor cell activity, and in combination with estrogen and progesterone, this increased stem cell activity is thought to be associated with an augmented Wnt signaling to serve as cell populations that are meant to be cancer cells of origin.[56] Through these findings, it can be easily deduced that prolactin and its receptor play a vital role in carcinogenesis, but there have been findings to have contrasted this idea: A study conducted using prolactin receptor neutralizing antibodies as monotherapy on breast cancer patients (75% of whom had ER+ subtypes of breast cancer), found no clinical significance of this treatment on disease progression.[57]
Some studies have found that inhibiting the effects of prolactin summarized above can enhance the sensitivity of breast cancer cells to other types of cancer medication, and rather not as an option for monotherapy.[58] It has to be also duly noted that, in the studies examined within the premises of this review, especially in regard to prolactin, a great heterogeneity among the sample characteristics was also present, where many important confounding factors, such as the age, body mass index, age at menarche and menopause for the patients enlisted, the duration of their postmenopausal period and hormone use varied widely, making it more difficult to draw conclusive results from a combination of studies showing consistent or contradicting results. As opposed to one study,[46] that had displayed certain data samples, indicating an apparent correlation in situ between higher prolactin levels in the serum and an increased risk of breast cancer, six other studies had data sets that were found to be demonstrating no correlation at all.[45,46,59-62]
In conclusion, breast cancer is surely a complicated entity posing crucial challenges for modern medicine still. Through our preconceived, textbook notion of understanding breast cancer, hormonal factors like estrogen and progesterone were already well established, even engraved into the classification of this disease. However, many pieces of modern medical research show other hormones like oxytocin and prolactin also to be playing roles just as important as those of estrogen and progesterone. Oxytocin is found to help relieve reactive oxygen species from the breast tissue, enhance the effects of exercise training on a treatment course and inhibit cell proliferation on cell lines such as T47D, MDA-MB-231, TS/A, and CMT-U27. It is also found to be driving cell migration, metastasis and facilitating EGF sensitivity, creating a microenvironment that promotes mammary tumor growth and metastasis. Prolactin on the other hand is demonstrated to facilitate DNA synthesis, epithelial cell proliferation, and the survival of cancer cells. It is found to be upregulated in up to 80% of all cancer types and it interacts with signaling pathways such as JAK2/STAT5, PI3K, and MAPK to influence cytoskeletal structure dynamics in the cell to interact with membrane-anchoring structures to finally allow tumor cells to arrive at local and distant metastases sights. Many epidemiological studies demonstrate higher levels of serum prolactin to be correlated with a higher risk for a patient getting breast cancer while locally elevated levels are also shown to contribute to disease progression. However, introducing prolactin inhibition as monotherapy for breast cancer patients is clearly revealed to be clinically ineffective, which goes out to show that none of these hormones examined during this study alone hold the keys to breast cancer treatment and cure. There is also a huge amount of research that still needs to be done on the role of oxytocin and prolactin in oncogenesis and disease progression, and these hormones still need to be better integrated into a complex mesh of algorithms that we use to define and approach the treatment of various types of breast cancer.